Process for the removal of silver nitrate stains

ABSTRACT

A process for the removal of silver nitrate stains from fabric after bleaching by maintaining at least a twenty-two-fold excess of thiosulfate ion over that of silver ion at a pH range of 5.0 to 13.0 and a temperature range of 50* to 150* F.

FIPBEB? 0R [451 Aug. 19, 1975 PROCESS FOR THE REMOVAL OF SI LVER NITRATE STAINS [75] Inventor: John D. Ciko, Allen Park, Mich.

[73] Assignee: BASF Wyandotte Corporation,

Wyandotte, Mich.

[22] Filed: June 21, 1973 [21] Appl. No.: 372,014

[52] U.S. Cl 8/109; 8/102 [51] Int. Cl. D06L 3/06; DO6L 3/08 [58] Field of Search 8/102, 108, 109

[56] References Cited UNITED STATES PATENTS 1,777,858 10/1930 Phair 8/109 2,727,804 12/1955 Lutgerhorst 8/109 2,938,764 5/1960 Blomfield 8/l08 3,434,796 3/1969 Colombo 8/109 OTHER PUBLICATIONS Encyclopedia of Chemical Technology 2nd ed., Vol. 20, p. 238.

Encyclopedia of Chemical Technology 2nd ed., Vol. 4, p. 920.

Primary ExaminerBenjamin R. Padgett Assistant ExaminerDonald P. Walsh Attorney, Agent, or FirmNorbert M. Lisicki; Bernhard R. Swick; Robert E. Dunn [5 7] ABSTRACT A process for the removal of silver nitrate stains from fabric after bleaching by maintaining at least a twentytwo-fold excess of thiosulfate ion over that of silver ion at a pH range of 5.0 to 13.0 and a temperature range of 50 to 150 F.

6 Claims, No Drawings PROCESS FOR THE REMOVAL OF SILVER NITRATE STAINS BACKGROUND OF THE INVENTION 1. Field ofthe Invention V The present invention pertains to a process for the removal of silver nitrate stains from fabric. More particularly, it pertains to a process for the prevention of the redeposition of silver unto the fabric. The process involves the steps of bleaching the fabric with a chlorine bleach, then treating the fabric with a solution of sodium thiosulfate wherein the concentration of thiosulfate exceeds that of the silver by a factor of at least twenty-two with the solution pH maintained between 5.0 and 13.0 at a temperature range of 50 to 150 F.

2. Prior Art One of the traditional methods for removing silver nitrate stains from fabric involves heavy bleaching with a chloro-hydantoin compound followed by neutralization and subsequent treatment with some polyoxide sulfur, e.g., sodium thiosulfate. US. Pat. No. 2,938,764 teaches that the bleaching effectiveness of aqueous l ,3- dichloro-l ,S-dimethylhydantoin solutions are significantly improved when the pH of the solution is in the range of 12 to 14. The patent also teaches that difficult stains are most effectively bleached when the bleaching composition contains an amount of alkalizing agent which results in a pH from 12.2 to 12.5. These techniques are effective for removal of the silver nitrate stain from the fabric, however, invariably the fabric after the completion of the wash did not have the desired degree of whiteness. It was discovered that the silver had a tendency to redeposit unto the fabric and actually restain the fabric.

SUMMARY OF THE INVENTION Now in accordance with the present invention, silver nitrate stains are effectively removed from the fabric by bleaching with a chlorine bleach solution and redeposi- .tergent composition and then the fabric may be treated with bleach and sodium thiosulfate.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the present invention, fabric stained with silver is cleaned by first bleaching the fabric with any suitable chlorine bleaching agent at a temperature of about 175 F., followed by treatment with a solution of sodium thiosulfate at a minimum concentration of at least twenty-two times that of the silver, preferably 22 to 130 times that of the silver. On a practical basis for industrial use, the concentration employed should be a minimum of 0.5 pound of sodium thiosulfate per 100 pounds of fabric. The preferred range is from about 0.5 pound to 4 pounds per 100 pounds of fabric.

Any of the various commercial chlorine bleaches may be used, such as sodium hypochlorite, trichlorocyanuric acid, dichlorocyanuric acid, or dichlorodimethylhydantoin. The preferred bleach is 1,3- dichlorol ,S-dimethylhydantoin. The amount of bleach which should be used is one which will give an available chlorine content of 750 ppm. to 1500 ppm., preferably from about 1000 ppm. to about 1250 ppm.

Any of the salts of thiosulfate may be employed. Among these are sodium, potassium and ammonium.

v The preferred salt is sodium thiosulfate at a concentration of at least 22 times that of the silver concentration. The temperature suitable for the prevention of redeposition may range from 50 to 150 F preferably to 1 10 F. Additionally, it was found that a suitable pH range is from about 5.0 to 13.0, preferably from about 7.0 to about 1 1.0. Ordinarily, the pH adjustment should be unnecessary since most commercial detergent compositions are alkaline and after several rinse cycles the pH of the water will range from 7.0 to l 1.0. In the event a pH adjustment is necessary, it may be readily accomplished by adding the required amount of a buffering agent such as sodium bicarbonate to raise the pH or-an acid silicofluoride compound may be used to lower the P a Fabric stained with silver nitrate may be reclaimed free of the silver without the use of detergent composition by the following procedure.

The fabric is treated in a washer with sufficient bleach solution such that the available chlorine is about 1000 ppm. for 20 minutes at a temperature of about 175 F. The fabric is then rinsed several times with cold water to reduce the rinse water temperature to about F. At this point about 2 pounds of sodium thiosulfate per 100 pounds of fabric is added to the rinse water. The fabric is agitated in the washer for about 5 minutes. Subsequently, at least three rinses of water at 100 F. should be employed. If insufficient rinse water is employed, the fabric will develop a yellow color upon treatment with laundry sour due to residual sodium thiosulfate on the fabric. If desired, the fabric may first be washed employing an alkaline detergent composition. After washing and suitable rinsing, the fabric may be treated with bleach and the subsequent steps followed as outlined above. The use of the above procedures has resulted in fabric which is essentially free of silver stains.

As it would be impractical to demonstrate on a commercial scale the optimum conditions for the preven-' tion of redeposition of silver stain unto fabric, a series of experiments were performed on a laboratory scale. Unbleached muslin swatches 2 /2 inches by 3 /2 inches were immersed in water containing sodium thiosulfate and silver nitrate. The pH was 9.0 and the temperature of the water was F. The immersion time was 20 minutes. The ratio of sodium thiosulfate to silver nitrate was varied from a factor of 0 to 140. The concentration of silver nitrate was 0.01 Molar. After the completion of the washing, the swatches were dried and the percent whiteness retention was determined by reflectance measurements using a blue filter in a Hunter Laboratory Model D-40 reflectometer manufactured by Hunter Associates Laboratory, lnc., Fairfax, Va. The percent whiteness retention was calculated from the ratio of reflectance readings of the cloth after immersion to the reflectance readings of the cloth before immersion. The results of this study are shown in Example 1.

EXAMPLE I Weight Ratio of Na S O to AgNO whiteness Retention The desired percent whiteness retention is a value over 90%. Fabric of whiteness retention less than 90% presents an appearance of uncleanliness. In a visual inspection of the fabric it is quite apparent that a fabric of 90% whiteness retention is appreciably whiter than one of 80% whiteness retention. The above data exemplifies the ratio of sodium thiosulfate to silver nitrate necessary to obtain the desired whiteness of the fabric.

EXAMPLE II The results of the tests carried out in the laboratory, described in Example I, were confirmed in practical washing operations using a G. A. Braun, Inc., washing machine. Approximately 100 pounds of fabric stained by silver nitrate were used.

Operation No. l was accomplished using 3 pounds of I,3-dichloro-5,S-dimethylhydantoin and 6 pounds of sodium chloride.

Operation No. 6 was accomplished using 1.5 pounds of sodium thiosulfate.

The fabric washed in this manner displayed no evidence of silver nitrate stains. Previous attempts to wash fabric similarly stained without using the process steps of this invention resulted in fabric displaying considerably Iess whiteness than the fabric washed as per Example II.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of removing silver nitrate stains from fabrics comprising a. soaking the fabric in an aqueous bath,

b. bleaching the fabric with a chlorine bleach,

c. rinsing the fabric with water substantially removing the bleach,

d. adding sufficient thiosulfate salt wherein the concentration of said thiosulfate exceeds that of the silver by a factor of from about 22 to about 130,

e. adjusting the pH to a range of about 5.0 to about f. maintaining the temperature of the bath from about 50 to about 150 F., and

g. effectively rinsing the fabric with water substantially removing the thiosulfate salt.

2. The process of claim 1 wherein the concentration of bleach is such as to give an available chlorine content from about 750 ppm. to about 1500 ppm.

3. The process of claim 1 in which the thiosulfate salt is sodium thiosulfate.

4. The process of claim 1 wherein the solution temperature is maintained from about to about 1 10 F.

5. The process of claim 1 wherein the pH of the solution is maintained from about 7.0 to about 1 1.0.

6. The process of claim 1 wherein the bleach employed is 1 ,3-dichloro-1 ,S-dimethylhydantoin. 

1. THE METHOD OF REMOVING SILVER NITRATE STAINS FABRICS COMPRISING A. SOAKING THE FABRIC IN AN AQUEOUS BATH, B. BLEACHING THE FABRIC WITH A KCHLORINE BLEACH, C. RINSING THE FABRIC WITH WATER SUBSTANTIALLY REMOVING THE BLEACH D. ADDING SUFFICIENT THIOSULFATE SALT WHEREIN THE CONCENTRATION OF SAID TIOSULFATE EXCEEDS THAT OF THE SILVER BY A FACTOR OF FROM ABOUT 22 TO ABOUT 130, E. ADJUSTING THE PH TO A RANGE OF ABOUT 5.0 TO ABOUT 13.0, F. MAINTAINING THE TEMPERATURE OF THE BATH FROM ABOUT 50* TO ABOUT 150*F., AND G. EFFECTIVELY RINSING THE FABRIC WITH WATER SUBSTANTIALLY REMOVING THE THIOSULFATE SALT.
 2. The process of claim 1 wherein the concentration of bleach is such as to give an available chlorine content from about 750 ppm. to about 1500 ppm.
 3. The process of claim 1 in which the thiosulfate salt is sodium thiosulfate.
 4. The process of claim 1 wherein the solution temperature is maintained from about 80* to about 110* F.
 5. The process of claim 1 wherein the pH of the solution is maintained from about 7.0 to about 11.0.
 6. The process of claim 1 wherein the bleach employed is 1,3-dichloro-1,5-dimethylhydantoin. 